This invention relates to glass-ceramic articles, especially glass-ceramic burner surfaces, decorated with a ceramic color and to a process for removing halos from glass-ceramic articles decorated with a ceramic color.
Glass-ceramic articles are widely used, for example, as kitchenware resistant to thermal shock and, especially, as cooking surfaces for stoves. These articles are, in general, provided with a decoration, either purely for reasons of appearance or, for example, to identify the hot cooking zones on the cooking surfaces. The colors used for the decorations are enamel-based ceramic colors applied by conventional techniques, such as, for example, burnt in (German Offenlegungsschrift 3,433,880; German Patent Specification 3,505,922; and German Patent Specification 3,600,109). As is known, glass-ceramics are produced from devitrifiable glass by heat treatment according to a defined temperature/time program (ceramization), temperatures of up to about 1100.degree. C. being reached. For production-engineering reasons and for the purpose of saving energy, efforts are made to carry out the burning-in of the decorative colors simultaneously with the ceramization, even though the ceramization temperatures are unusually high compared with conventional burning-in temperatures (German Patent Specification 3,505,922).
Because of the high burning-in temperatures, numerous ceramic colors are unsuitable for decoration. With the remaining colors, halos often form in the shape of bleeding, blurred contours of the burnt-in decoration (hereafter called "halozation"). The resultant glass-ceramic articles cannot be used as high quality products and represent was or heavily discounted products.
German Patent 3,936,654 C1 has disclosed a process for producing a decorated glass-ceramic article, wherein the occurrence of halozation is avoided. This process comprises providing, before the decoration, an interlayer of SiO.sub.2 on the surface of the glass-ceramic article which is to be decorated. The decoration is printed onto the interlayer and no longer shows any tendency to form a halo. Even though the formation of a halo can be reliably avoided by this process, it nevertheless still has some disadvantages. For example, all of the glass-ceramic articles to be decorated must, in fact, be coated with a layer of SiO.sub.2 before decorating, even though the formation of a halo may occur only sporadically.